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As a seasoned KSP player, I am aware of how important it is during ascent to have center of thrust and center of mass perfectly aligned. If you don't, your rocket will deviate from its course and finally crash.

The Shuttle seems like a pretty obvious asymmetric launch stack. I assume that at liftoff balance was perfect, but how was it maintained when the big tank loses more and more mass? How about the boosters? When they were jettisoned, wouldn't that mean a rather dramatic shift in balance?

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a good attempt by scott manley, with a bunch of explanation on the thrust vectoring capabilities of the various engines used: youtube.com/watch?v=otv7lJmLJtQ – njzk2 Mar 3 at 15:23
    
@njzk2 this tutorial is a tad obsolete, today all you need is Vector, which is KSP equivalent of SSME. (nevertheless, it have some good information) – PTwr Mar 3 at 16:45
up vote 16 down vote accepted

The variations in the centre of mass was handled by the huge gimbal range of over 20 degrees.

gimbal

Also, the heaviest part of the propellant, the liquid oxygen, was placed in the upper part of the external tank. That means that the centre of mass was pretty high, placing it far from the engines, and thereby reducing the deviation angle.

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How did they know the precise angle at any given situation? Was that precalculated or is there some way to measure the current center of mass? – choeger Mar 3 at 10:29
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@choeger: You don't need to know it very precisely, a rough estimate is fine. Add a sensitive sensor to measure the tilting of the whole stack and then correct for that. This sensor is needed either way, as gravity and engines are not the only sources of force: wind and air drag also need to be corrected for. – asdfex Mar 3 at 11:44
    
@choeger The shuttle monitored pitch, yaw and roll during launch, and that was fed to the Main Engine Controller (MEC). – Hohmannfan Mar 3 at 12:14
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I should have thought about that. Its not necessary to measure the cause when you can measure the effect. – choeger Mar 3 at 13:02
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@Hohmannfan , not the Main Engine Controller. That played no role in thrust vectoring. That was all the onboard computers (GPCs). SSMEC (Space Shuttle Main Engine Controllers) just monitored the engines and passed commands and data back and forth. No involvement in TVC. science.ksc.nasa.gov/shuttle/technology/sts-newsref/… – Organic Marble Mar 3 at 14:27

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